Identification systems are known in which a plurality of transmitters, typically transponders (commonly called tags), are activated by a power signal (or an “interrogation signal”) and then transmit signals, usually containing identification data to a receiver, which typically forms part of the interrogator. The signals may be transmitted in many ways, including electromagnetic energy, eg. radio frequency (RF), infra red (IR), coherent light and sound, eg. ultra-sound. For example the transmission may be achieved by actual emission of RF energy by the transponders, or by the modulation of the reflectivity of an antenna of the transponder, resulting in varying amounts of RF energy in the interrogation signal being reflected or back-scattered from the transponder antenna.
Radio Frequency Identification systems are used to remotely identify, take a census of, locate or otherwise interact with people, objects or groups or clusters of people or objects. The systems usually comprise interrogators also known as readers, and transponders also known as tags.
It is not usually a problem for a reader to communicate with a single tag which is presented to the reader, such as in an access control system. However in the situation where many tags may be present in a reader's field of views, such as a crowd of people, or a pallet load of goods having tags attached, the transmissions by the tags would occur together and cause collisions, rendering the transmissions unusable due to mutual interference. A number of arbitration methods have been developed to enable a reader to sort and/or isolate and transact with these large populations of tags. These methods are known variously as anti-collision schemes or collision-arbitration algorithms.
In one example described in U.S. Pat. No. 5,537,105 (corresponding to EP 494114 B1) by Marsh et al, the whole contents of which are incorporated herein, the transponders on receipt of an interrogation signal repeatedly transmit a response signal containing data which identifies the transponder. The interrogator detects successful identification of any transponder and briefly modifies the interrogation signal to indicate the successful identification. Each transponder includes a logic circuit responsive to a respective modification in the interrogation signal to cease transmission of its own response signal. The response signals are transmitted at random intervals until the identity of a transponder is successfully read and acknowledged by the reader and placed into a dormant or gagged state. U.S. Pat. No. 5,699,066 (corresponding to EP0585132) and PCT application GB98/01385 (corresponding to WO/985142) also describe methods in which the response signals are transmitted at pseudo-random intervals. The whole contents of EP0585132 and WO/985142 are incorporated herein by reference.
Other examples of such methods are described in U.S. Pat. No. 5,699,096, US 2003067414 (corresponding to Cole WO 01/41043 A1), and Maletsky U.S. Pat. No. 6,104,279.
Methods have been used to improve the randomness of the response intervals. In U.S. Pat. No. 5,528,232, U.S. Pat. No. 5,640,151, U.S. Pat. No. 5,686,902 and U.S. Pat. No. 5,973,613 (all corresponding to EP 467036 B1), the whole contents of which are herein incorporated, the identification system uses a pseudo-random delay between transponder data transmissions. In this example, a linear recursive sequence generator is seeded by the transponder identification address to provide the pseudo-random delay between tag data transmissions. U.S. Pat. No. 5,550,547 describes a similar system in which the tag sends out a 64 bit ID code at intervals determined by a random number generator. U.S. Pat. No. 6,104,279 describes a system in which remote units re-transmit their bit pattern at random intervals. It further mentions that there are many techniques to produce a random number; for example the identification number can provide the seed for a random number generator permitting the user to individually seed each tag with a different random number.
Another method is based on slotted polling or slotted Aloha schemes in which tags randomly select a time slot in which to transmit and then transmit when it is their turn to do so. The theory is that because slots are randomly selected, sooner or later all tags will have had the opportunity to transmit messages ‘in the clear’. WO 01/41043 describes such a system in which RFID tags randomly select a slot in which to transmit. In a practical implementation the slot selection by a tag is made on a pseudo-random basis, using a seed for a random number generator, which is derived from either part of the data held on the tag or by pre-programming a seed where the tag is manufactured. The possibility is great that many tags will have the same slot allocation choice. The fewer the number of slots to choose from, the greater will be the probability that many tags will ‘randomly’ select the same slot time and so will always collide and will therefore never be successfully read.
European patent EP0983569B1 the contents of which is included herein, describes an arbitration method similar to that in U.S. Pat. No. 5,537,105 with improvements in the form of a Mute command and functionality which provides a method of arbitration based on Carrier Sense Multiple Access (CSMA) as used in Aloha communications systems such as IEEE 802.11 and Ethernet. In this system, the interrogator listens for a first transmission from a first tag (transponder) and on detecting this first transmission the interrogator issues a Mute command which causes all other tags present in the field to freeze or go into a suspended condition while the interrogator completes reading the identity or data from the first tag. Once the first tag has completed its reply and the interrogator has successfully finished collecting the data, the interrogator issues an Acknowledge command to the first tag causing it to move to a quiet or sleep state, thereby removing it from the active tag population. After removing it the first tag from the population, the interrogator causes the tags in the suspended state to return to the active state, and it continues to collect data from these tags using the same process, until all tags have been successfully read and acknowledged. The command to return the remaining tags to the active state may either be combined with the Acknowledge command (as a single command) or it may be a separate command which is issued by the interrogator after it has sent the acknowledge to the first tag.
Another method described in used to improve the throughput of RFID systems using Aloha arbitration is by dynamically adjusting the maximum hold-off time or the number slots in an arbitration round. As congestion increases the maximum hold-off time is increased or number of slots is increased to permit more time for individual tags to reply. As the congestion decreases and there is unused airtime, the maximum hold-off time is decreased or the number of slots is decreased in order to maximise reading efficiency. U.S. Pat. No. 6,784,787 (corresponding to Patent Application WO99/26081), the whole of which is included herein by reference, describes a method and system for improving the reading efficiency of slotted systems by optimising the maximum waiting time before a tag (transponder) transmits or adjusts the number of slots over which a tag may randomly choose to transmit its reply. If there are too many tags in the field and the congestion is heavy, the maximum waiting time may be increased or the number of slots may be increased to relieve this congestion. The converse is also true; if there are few tags in the field and there is little or no congestion, the maximum waiting time may be decreased or the number of slots may be decreased in order to improve overall throughput. A Tag (Transponder) may dynamically alter the waiting time or number of slots over which it randomly transmits in response to an instruction from the interrogator. Alternatively, the tags (transponders) may be adapted to detect either heavy or light congestion and adjust their waiting time or slot number accordingly. Tags may increment or decrement their wait time or number of slots in increments in a number of stages or alter to any length for example number of clock pulses. The alteration of the waiting period imposes no limitation on the technique which may be used to determine the waiting time or number slots. When referring to time slots, a number of time slots are arranged in groups representing the maximum waiting time. These groups are usually referred to as Rounds. The number of time intervals contained within a maximum hold-off period can therefore be referred to as the Round Size.
A weakness of systems which attempt to arbitrate and read the identity or data in all tags present, is the need for the reader to systematically conduct an inventory and collect the data from all the tags present, whether their data or identity is required or not. For example in the case of a pallet load of goods; if the pallet contains a mixture of grocery items such as coffee, sugar, tinned foods and beer, and the reader wishes to conduct an inventory of only the beer, then it needs to conduct an inventory of the entire pallet, then on completing the inventory, discard the identities or data belonging to the unwanted items. This results in more “over the air” congestion than is necessary to take an inventory or collect data from the wanted tags and increases the read time proportional to the number of unwanted items (tags) present. In known systems, such as those described in the ISO/IEC 18000 air interface standard for item management, a number of methods are used to thin the population present so that arbitration can take place more quickly and efficiently. One such method is the “AFI” or Application Family Identifier”. The arbitration initialisation command contains a parameter with an AFI value. Only those tags whose AFI matches the parameter will participate in the arbitration. This AFI parameter in the command is an 8 bit number of which only 16 values are available for item management, the balance being allocate to contactless smart cards. Another method uses specific “group select” commands, such as those described in the ANSI NCITS T6 standard, to systematically select groups or sub-groups of tags so that they may participate in the arbitration and identity or data collection process. The group selection mechanism requires the group identity to be held in a register on the tag. There is no provision in the select mechanism to exclude tags or groups of tags from the inventory process. Another group select mechanism is used in the Auto-ID Center EPC Class 1 air interface specification. In this specification, tags may be selected according to selection criteria which match a portion or all of the data on the tag. However with this method only one selection may be made per inventory process and there is also no means to exclude individual tags or groups of tags from the inventory process.
There are disadvantages to the known systems. One of these is the simplistic nature of the AFI, which only provides a very course or limited range of selection. Another disadvantage is the complex logic required on the transponders to accommodate group select mechanisms. One feature completely missing from known systems is the ability to exclude groups of transponders; in the example of the pallet of groceries discussed above this could be to exclude coffee and sugar while taking an inventory of all other items. Another problem with many known systems is the inability to select transponders for arbitration based on the data content held within the memory of the transponder itself or that the selection mechanism requires complex logic circuitry. Complex logic circuitry results in higher current consumption than simple logic, higher current consumption requires that the transponder have a larger DC storage capacitor resulting in a larger die area which directly impacts cost of the transponder. Higher current consumption also has the effect of reducing the communication range of the transponder which is disadvantageous to the user of the RFID system using these transponders.
EP 0702 323 A2 describes a method of communication with subsets of a group of RFID tags. The method has provision to move tags between a ready state and a selected state, and thereafter performing operations on the tag(s) which may be by way of arbitration or reading of identity or data from the tag(s). The selection mechanism employed in EP 0702 323 A2 changes the state of the tag's state machine requiring complex logic circuitry and the selection procedure is not versatile.
The present invention strives to at least alleviate many of the disadvantages of the known systems and provides a simple but powerful mechanism for selecting tags for arbitration. The present invention also strives to provide a far more versatile selection solution, that may be used with almost any type of state machine, from a simple one to a largely complex one.